A typical mass spectrometer includes an ion source module, an analyzer module, and a detector module. Typically, in the ion source module, a sample that is to be analyzed is converted to an ion beam comprising a plurality of charged particles dispersed in a carrier gas. The analyzer module separates the charged particles according to their mass using electromagnetic fields. The detector module then measures or calculates the abundance of various types of ions to provide information for determining the composition of the sample.
Usually, the ion source module operates at or near atmospheric pressure (e.g., in the case of atmospheric-pressure ionization (API) and atmospheric-pressure chemical ionization (APCI) ion sources). The analyzer module, however, usually requires high vacuum conditions to operate effectively. To transport ions that are generated at atmospheric pressure and contained within a carrier gas into a vacuum region, an orifice plate typically is used that defines a sampling orifice through which the ions are able to pass. The sampling orifice is generally quite small (e.g., 1 mm in diameter) to enable maintenance of a high pressure differential across the orifice plate. In addition, the analyzer module can include several stages of differential pumping to create large pressure differences, in which case each of a plurality of pressure regions are connected in series through apertures in order to allow gas flow from one pressure region to the next. Due to the limited size of the sampling orifice and the apertures between each pressure region, a significant portion of the dispersed ions fail to pass through such openings. As a result, many ions of interest are lost, and the overall sensitivity of the system is reduced.
A number of systems have been proposed to focus an ion beam towards an opening so as to increase the proportion of ions that pass through the opening and thereby improve system sensitivity. For example, a radio frequency (RF) ion funnel has been proposed that includes a plurality of axially-aligned ring-shaped electrodes mounted in proximity to an opening through which the ion beam is to be directed. Each electrode has an inner diameter that is less than that of the immediately-preceding electrode, such that ions travelling in the axial direction towards the opening encounter electrodes having progressively-smaller inner diameters. RF potentials are applied to each of the electrodes, and the phase of the RF potential is varied from one electrode to the next. Ions travelling towards the periphery of the RF ion funnel are repelled by the field existing near the surface of the electrodes, and are thereby urged towards, and substantially confined to, the essentially field-free center region of the RF ion funnel. Meanwhile, much of the carrier gas is permitted to escape the RF ion funnel through spaces between adjacent electrodes.
While RF ion funnels have shown some promise, they suffer from a number of shortcomings. Most notably, the ability of such systems to effectively confine ions deteriorates rapidly as operating pressure increases. Thus, if the sampling orifice or apertures between pressure regions are increased in size to allow more ions to pass, the operating pressure quickly reaches a point at which the RF ion funnel becomes ineffective. In addition, RF ion funnels require a large amount of RF power to operate, especially when higher RF frequencies are used to allow the system to operate at higher pressures.
Accordingly, a need exists for improved systems and methods for focusing dispersed ions.
Further introductory information can be found in the following references, the entire content of each of which is incorporated herein by reference:
U.S. Pat. No. 6,107,628 to Smith et al., entitled “METHOD AND APPARATUS FOR DIRECTING IONS AND OTHER CHARGED PARTICLES GENERATED AT NEAR ATMOSPHERIC PRESSURES INTO A REGION UNDER VACUUM”;
U.S. Pat. No. 6,639,213 to Gillig et al., entitled “PERIODIC FIELD FOCUSING ION MOBILITY SPECTROMETER”;
U.S. Pat. No. 7,223,969 to Schultz et al., entitled “ION MOBILITY TOF/MALDI/MS USING DRIFT CELL ALTERNATING HIGH AND LOW ELECTRICAL FIELD REGIONS”;
U.S. Pat. No. 7,259,371 to Collings et al., entitled “METHOD AND APPARATUS FOR IMPROVED SENSITIVITY IN A MASS SPECTROMETER”;
U.S. Pat. No. 7,365,319 to Hager et al., entitled “METHOD FOR PROVIDING BARRIER FIELDS AT THE ENTRANCE AND EXIT END OF A MASS SPECTROMETER”;
U.S. Patent Publication No. 2010/0038532 to Makarov et al., entitled “EFFICIENT ATMOSPHERIC PRESSURE INTERFACE FOR MASS SPECTROMETERS AND METHOD”;
GERLICH, “Inhomogeneous RF Fields: A Versatile Tool for the Study of Processes with Slow Ions,” Advances in Chemical Physics Series, Vol. LXXXII (1992);
GUAN et al., “Stacked-Ring Electrostatic Ion Guide,” Journal of American Society for Mass Spectrometry 1996, 7, 101-106; and
KREMER et al., “A Novel Method for the Collimation of Ions at Atmospheric Pressure,” Journal of Physics D: Applied Physics 39 (2006) 5008-5015.